The exchange of cross-talks between cells relies on soluble factors or direct cell-cell contact. Soluble factors increase the expression of cell surface molecules that activate adjacent cells by direct contact to produce cytokines. In the lung, dendritic cells are potent inducers of T-cell proliferation, and the interaction between the two leads to the production of high amounts of TNFα and TNFβ. Of the molecules involved in these biologic functions, LFA-3, CD11c, and the combination of β1 and β2 integrins are the most efficient. However, blocking TNFα or TNFβ production does not affect the alloreaction. The interaction between activated T cells and monocytes resulted in a large production of IL-1β. In this reaction, CD69, CD2, and the β2 integrins (CD11a, b, c, and CD18) and also other molecules such as a 25- to 35-kD glycoprotein play an important part. Finally, interaction between monocytes and fibroblasts leads to the production of large amounts of collagenase and PGE2 by fibroblasts. Cell-associated IL-1, particularly IL-1± and membrane-bound TNFα, can also play a crucial role in the process of cell-cell interaction. This interaction may be controlled by inhibitors to IL-1 and TNF.